Chances of prolonged La Niña more than 50%

Weather expert sees lengthy water shortage

La Niña

La Niña occurs when there’s a prolonged cooling of the sea surface in the Central Pacific Ocean. During La Niña, the jet stream, which carries most storms across the globe, tends to target the Pacific Northwest, leaving Southern California out of the storm track for most of the winter. Northern California is in the middle; sometimes it gets the storms, sometimes it misses them.

Rain-robbing La Niña will not only visit this winter, but it may hang around like an unwanted house guest for another year — or longer. If it does, it could prolong water shortages for Southern Californians, many of whom are already saddled with restrictions on when and how much they can use their sprinklers, or wash their cars.

Klaus Wolter, a long-range forecaster who consults with the California Department of Water Resources to help set water-management strategies, said the current La Niña is one of, if not the strongest La Niña on record. The stronger the La Niña, the more likely it will last, he said.

“The odds are quite high that we won’t see a short-lived La Niña,” Wolter said in San Diego Wednesday. “The odds are higher than 50-50 (that it will continue).

“La Niña is fundamentally different (from El Niños). Events (of this size) have lasted two, even three years.”

La Niñas are marked by extended periods of below-normal sea-surface temperatures in a large swath of the equatorial Pacific Ocean. Those cool waters tend to change the path and intensity of winter storms. In a typical La Niña, the Pacific Northwest is wetter than normal, while Southern California stays dry.

Ironically, wet weather is forecast for the region this weekend. National Weather Service forecasters expect a half inch to an inch of rain near the coast by Monday. The rain should come in two pulses, the first Saturday morning or afternoon, the second Sunday afternoon or evening. Showers could continue through Monday morning. The mountains could get 2 to 2.5 inches of rain.

Wolter, who spoke at the Westin Hotel in the Gaslamp Quarter during a workshop on the winter outlook sponsored by the DWR, said the rain this weekend and the abnormally wet October in California and the Colorado River Basin, another major source of imported water for the region, will have little bearing on the coming months.

“As much as we rejoice with all this moisture, it doesn’t mean much for the rest of the winter,” said Wolter, who works for the National Oceanic and Atmospheric Administration’s Western Water Assessment Team and the University of Colorado. “The winter season is really what counts.”

December, January and February are the key months in California, because 50 percent of the state’s precipitation typically falls then. Wolter expects a dry winter in Southern California, with about even odds for a normal winter in the northern end of the state.

Historically, strong La Niñas have led to very dry winters in San Diego. The city averaged 6.33 inches of rain during the five strongest episodes since 1950. The 30-year annual average is 10.77 inches.

The year following the onset of a strong La Niña has also been drier than normal in San Diego, but on average, not quite as dry as the first year. The second La Niña year has had a bigger impact on parts of Northern California, which tend to experience reduced river runoff, Wolter said.

Beyond early next week, Southern California could fall into a long period of little or no rain, according to Jim Purpura, meteorologist in charge at the National Weather Service’s Rancho Bernardo office. That is what often happens during La Niña years, he said.

“We tend to have long dry periods, broken by five- to 10-day periods with heavy rain,” Purpura said.

Jeanine Jones, interstate water resources manager for the DWR, said the state’s reservoirs are not in bad shape as winter begins. The 2010 hydrologic year, which concluded Sept. 30, was about average for most of the state after three straight dry years.

However, pumping restrictions in the Sacramento-San Joaquin Delta continue to make it difficult to ship water from the north to Southern California, which imports the vast majority of the water it consumes. The southern end of the state has not received its full allotment of water from the state since 2006. No reductions on water deliveries from the Colorado River are expected this year.

If La Niña extends beyond this year, the biggest impact could be on rural county residents who rely on wells. A multiyear drought would deplete groundwater supplies.